Dendritic cells constantly sample the surroundings for viruses and bacteria. Once they have captured such an invader, they cut its proteins into small pieces and present those fragments at their cell surface using MHC molecules. They then travel through the blood stream to the spleen or through the lymphatic system to a lymph node. Here they act as antigen presenting cells: they activate helper T-cells and killer T-cells as well as B-cells by presenting them with the pieces of the invader. Depending on the type of invader, this results in an immune response involving antibodies or killer cells.
Every helper T-cell is specific to one particular antigen. Only dendritic cells are able to activate a helper T-cell which has never encountered its antigen before.
Dendritic cells form from monocytes, white blood cells which circulate in the body and, depending on the right signal, can turn into dendritic cells or macrophages. The monocytes in turn are formed from stem cells in the bone marrow.